1. Field of the Invention
The invention relates to an electronic pressure switch for only two supply leads, with a resistance measuring bridge as a pickup, with an amplifier unit connected downstream of the resistance measuring bridge, with a comparator connected downstream of the amplifier unit and with a switching stage connected downstream of the comparator.
2. Description of Related Art
The above described type electronic pressure switch serves as the starting point for the invention, and will be explained with regard to the following aspects.
First of all, the subject matter of the invention is an electronic switching device which switches, i.e., passes from the non-conductive state into the conductive state or from the conductive state into the non-conductive state when the pressure in a liquid or a gas exceeds or fails to the reach a given threshold value. Therefore, a pressure switch can also regarded as a digital pressure measuring device. While an analog pressure measuring device at its output provides an output signal which is proportional to the measured pressure, therefore an output voltage proportional to the measured pressure or an output current proportional to the measured pressure, a pressure switch as a digital pressure measuring device delivers only two different output signals, specifically “measured pressure above a given value” or “measured pressure below a given value”.
What is important for the electronic pressure switch to which the invention relates is that it is designed for only two supply leads, and therefore, has only two terminals for only two supply leads. Therefore, it is an electronic switching device which can be connected via an outer conductor to one pole of an operating voltage source and only via another outer conductor to the terminal of a consumer or of a control circuit, and the other terminal of the consumer or of the control circuit is connected or can be connected to the other pole of the operating voltage source.
Electronic switching devices of the type under consideration have been used for decades instead of electrical, mechanically actuated switching devices which do not have a solid state construction. In terms of their operation as switching devices, almost no residual current will flow in the electronic switching devices under consideration in the blocked state and almost no voltage drop will occur in the conductive state. However, if no residual current were to flow in switching devices of the type under consideration in the blocked state, no supply current could be obtained for the circuit provided within the switching device, and if in the conductive state no voltage drop were to occur, no supply voltage for the circuit within the switching device could be obtained either, it applies to all electronic switching devices for only two outer conductors that, in the blocked state, a residual current flows and in the conductive state a voltage drop occurs. This results in that, even if unintentionally but functionally necessary in the blocked state, a residual current flows and in the conductive state a voltage drop occurs, the residual current and the voltage drop will be as small as possible. This problem has already been extensively treated.
It was stated initially that the electronic pressure switch under consideration includes a resistance measuring bridge as a pickup. Instead of a resistance measuring bridge, another circuit can also be used with which a measurement signal proportional to the prevailing pressure can be generated. In particular, the pickup can also be formed of GMR elements (i.e., giant magneto resistance elements). While an electronic pressure switch with a resistance measuring bridge as the pickup is described below, the teaching of the invention is not limited thereto; it can also be easily applied to pressure switches with another type of pickup, especially pressure switches in which the pickup is formed of GMR elements, also to electronic switching devices with other pickups, for example, position sensors in which the pickup is formed of GMR elements.
An electronic pressure switch for only two supply leads which on the market from the company UNITED ELECTRIC CONTROLS, 180 Dexter Avenue, Watertown, Mass. 02471-9143, USA, and is also the subject matter of patent publications, specifically U.S. Pat. No. 6,522,249 and PCT patent application publication WO 03/021,195. This electronic pressure switch, in the blocked state, has a residual current of roughly 0.75 milliamps, and in the conductive state, a voltage drop of up to roughly 7 V. These are relatively large values. The residual current of roughly 0.75 milliamps is critical, for example, in higher resistance applications in input circuits of optical couplers because, there, the blocked state can no longer be recognized. The voltage drop of up to roughly 7 volts in the switched state greatly limits applications, since the operating voltage, for example, for a consumer, is reduced by roughly 7 volts. Furthermore, the maximum switched current must be limited because a heat loss results from the product of the switched current and the voltage drop.